1. Field of the Invention
The present invention relates to the field of injection molding. More particularly, the invention relates to an insulator that covers part of the exterior of a hot runner nozzle, or to the nozzle tip on the end of the injection molding machine injection unit. The insulator is held onto the nozzle or nozzle tip by a removable fastening structure.
2. Description of the Related Art
Insulators are used to thermally insulate the heated tip or nozzle housing of a hot runner nozzle assembly from the surrounding cooled gate insert of a mold cavity. U.S. Pat. No. 4,662,837 to Anderson (incorporated herein by reference) shows such an insulator assembly. FIGS. 3-5 inclusive show an insulator assembly comprising two elements 41 and 42 that thermally insulate the housing end 15 from the gate insert 20. The insulators are typically made of a high temperature resistant, resilient material such as "Vespel". The insulators occupy the space that would otherwise be filled with a resin "bubble", well known in the art. If the resin being processed tends to degrade over time and it is allowed to form the insulating bubble around the tip the degraded resin eventually is drawn into each part being molded causing unsatisfactory properties and appearance. It is therefore essential that the insulating function of the bubble be performed by some material other than the resin being processed if it is thermally sensitive or if plastics of differing colors are to be processed and errant color streaks are undesirable.
The insulators in the '837 patent are not fastened to the housing and when the housing is separated from the gate insert they could remain in the insert 20. The insulators must then be removed for cleaning or replacement prior to reassembly of the housing, and this removal can be time consuming and problematic because it requires dismantling the mold. This process is not conducive to making repairs in a production environment.
U.S. Pat. No. 5,208,052 to Schmidt et al. (incorporated herein by reference) shows another insulator construction that surrounds a heated tip of a hot runner nozzle assembly. FIG. 2 shows insulator 32 surrounding tip 31 and extending toward the gate sufficiently to displace most of the bubble in the gate area. The insulator 32 is made of titanium and separated from both the tip 31 and cooled gate area 22 by air gaps on either side. The insulator is threaded onto the nozzle housing 30 and retains tip 31 in said housing by means of the contact area 50. The tip must seal against the bubble wall to prevent resin leaking behind it and occupying the air gap space. Sealing is effected by seal 34 in combination with deflection of the insulator leg pressing between the tip at 50 and the bubble wall. This design has the disadvantage of being relatively costly and titanium is not as effective as a thermal insulator as "Vespel".
U.S. Pat. No. 5,639,490 to Travaglini is an example of a means to prevent a hot runner nozzle tip from loosening while in operation. An axially elongated bridging key arrangement is used to lock the nozzle tip to the nozzle housing.
Co-pending U.S. application Ser. No. 09/170,054 to Jenko (incorporated herein by reference) shows a two piece insulator that surrounds the tip. An inner titanium sleeve is surrounded by an outer "Vespel" sleeve and the assembly is slid over the tip and held in place by the bubble shape in the gate area. Releasably fastening the insulator to the tip is not taught.
It is therefore desirable to have an effective thermal insulator that can be easily attached and removed from the nozzle tip to facilitate ease of maintenance during production operations.